1. Field of the Invention
The invention relates to semiconductor wafer processing systems and, more particularly, the invention relates to a method and apparatus for discharging an electrostatic chuck used to retain a semiconductor wafer in such a semiconductor wafer processing system.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer processing system. Although electrostatic chucks vary in design, they all are based on the principal of applying a voltage to one or more electrodes embedded in the chuck so as to induce opposite polarity charges in the workpiece and electrode(s), respectively. The electrostatic attractive force between the opposite charges pulls the workpiece against the chuck, thereby retaining the workpiece.
A problem with electrostatic chucks is the difficulty of removing the electric charge from the workpiece and the chuck when it is desired to release the workpiece from the chuck. One conventional solution is to connect both the electrode and the workpiece to ground to drain the charge. Another conventional solution, which purportedly removes the charge more quickly, is to reverse the polarity of DC voltage applied to the electrodes. This technique is described in the context of a chuck having two electrodes (a bipolar chuck) in U.S. Pat. No. 5,117,121 issued May 26, 1992 to Watanabe, et al.
A shortcoming that has been observed with these conventional approaches to removing the electric charge is that they fail to completely remove the charge, so that some electrostatic force remains between the workpiece and the chuck. This residual electrostatic force necessitates the use of a large mechanical force to separate the workpiece from the chuck. When the workpiece is a semiconductor wafer, the force required for removal sometimes cracks or otherwise damages the wafer. Even when the wafer is not damaged, the difficulty of mechanically overcoming the residual electrostatic force sometimes causes the wafer to pop off the chuck unpredictably into a position from which it is difficult to retrieve using a conventional wafer transport robot.
To more accurately reduce the residual electrostatic attractive force that remains between the workpiece and the chuck, attempts have been made to optimize the dechucking voltage by performing measurements upon the chucked wafer to determine an optimal dechucking voltage. One such active dechucking arrangement is disclosed in commonly assigned U.S. Pat. No. 5,459,632 issued Oct. 17, 1995 to Birang, et al. incorporated herein by reference. This dechucking method, like most optimizing type dechucking methods, optimizes the magnitude of the dechucking voltage. As such, these systems may attain an optimal dechucking voltage for a given dechucking period, but this combination of dechucking voltage and dechucking period may not be optimal for dechucking the wafer.
Additionally, when successively processing a plurality of workpieces, previous chucking/dechucking methods apply the same polarity voltages for each chuck/dechuck cycle. This leads to chuck dielectric polarization and an accumulation of residual charge on the chuck surface. The result is increasing difficulty in dechucking each successive wafer.
This accumulation of the residual charge is also detrimental to chucking of the wafer. The accumulated charge interferes with the chucking voltage by either being additive, if the accumulated charge has the same polarity as the chucking voltage, or is subtractive, if the accumulated charge has an opposite polarity as the chucking voltage.
To decrease the residual charge, the chuck can be utilized at high temperatures. In certain ceramic chucks, using the chuck at a higher temperature (e.g., greater than 200.degree. C.) makes the chuck material more conductive. As such, some of the residual charge will dissipate by conduction through the chuck to the electrodes as long as the chuck is maintained at the high temperature.
Therefore, there is a need in the art for a method to discharge the residual charge on the electrostatic chuck without relying on a high temperature to induce conduction.